Gas Hot Water Boiler NGC

[Kiwi_Roy]

Our 30 Unit apartment block has a hot water circulating system that is playing up.
I'm sure we have an expert on these systems, would you mind giving me the benefit of your knowledge?
Thanks in advance.

AO Smith HW 399 200 copper tube boiler with a VR8304P4322 gas valve controlled by Honeywell S8610U controller
At the moment it's heating up to the High Limit and cutting off.
When it cools back down and the High Limit thermostat closes the pilot gas valve just buzzes but won't open enough to establish a pilot flame, spark looks ok.
If I close the manual shut off on the gas valve then slowly open it again the pilot will open (no buzz) followed a second or two later by the main valve opening. This only happens if you are standing there, with no intervention we end up with cold water.
I checked the Voltage across the pilot valve coil while its buzzing 25 VAC (same as when I get it open)
These valves seem to get a really good workout, something like 30seconds on 30 seconds off although it will stay on much longer when the system has been down for an hour or two.

We have been without hot water over the long weekend, I got up early this morning and nursed it through the heat up process so the residents can take a shower.

Roy

[RinkRat II]

       Sounds as though the pilot portion has gotten gummed up and is not closing all the way. Then when called for it's staying closed due to the gumminess. Can you take that portion apart and clean the residue out and check the spring in the valve.  Nat gas or propane?  Drip leg full of gunk too?  My $.02

       Paul B 

[Kiwi_Roy]

       Sounds as though the pilot portion has gotten gummed up and is not closing all the way. Then when called for it's staying closed due to the gumminess. Can you take that portion apart and clean the residue out and check the spring in the valve.  Nat gas or propane?  Drip leg full of gunk too?  My $.02

       Paul B 

Gumminess eh. I never thought of that but I'm not a gas fitter. Natural Gas.
I'm a bit reluctant to take it apart in case I screw it completely but I can drop the bottom off the drip leg for sure.
The plumbing contractor has been called, they quoted us $1700 for a new valve but don't have any on hand.

[RinkRat II]

      Look like this....... Alittle cheaper and probably in stock.https://www.grainger.ca/en/product/GAS-VALVE%2C3-4%22-FPT/p/RSC700-515?searchBar=true&nls=NLSAA_11-1

      Paul B

[Kiwi_Roy]

      Look like this....... Alittle cheaper and probably in stock.https://www.grainger.ca/en/product/GAS-VALVE%2C3-4%22-FPT/p/RSC700-515?searchBar=true&nls=NLSAA_11-1

      Paul B

Hi Paul,
           Am I correct in thinking this is something you do for a living?
Yes that looks like it would be a good equivalent, but I dont have any say in who they hire to replace the parts, I only got involved reluctantly after 3 days without a shower LOL, some of the contractors treat us strata properties as a cash cow.

After I got the unit up to temperature i find it is still running, I suspect it is missing a few times then finally igniting. Last time I was there it buzzed for about 10 seconds then went plunk as the pilot opened.
What has me puzzled is when it turns on the pilot is buzzing at 25 VAC but there is also 13.5 Volts across the main valve coil.
As soon as the pilot opens (perhaps a second delay) then the main valve jumps to 25 V and the main flame comes on.

What do you thing of the rapid On/Off cycle. the High Limit is set at 140F up at out apartment on 4th floor we are running 120F
The cycle time surprised me but I'm not used to these small systems, most of what I have worked on are large dryers or open flame mine heaters.

I will go and drop the trap off in a minute.

Thanks for your input thus far
Roy

[RinkRat II]

        After looking up your boiler, it appears the valve in question requires a slow opening valve which is dictated internally  by a light spring which the pilot circuit must overcome, probably why you only see 13.5 volts. The other possibility is the pilot coil is weakening.  I'm still leaning towards gummy residue  as nat gas is somewhat dirtier than propane.  Over the years I've worked on controls for pools, hot tubs and furnaces and learned from all of them. As they say,start with the easy crap first. 
   Is that a 3/4 or 1" valve?    As  far as cycle time is concerned it does seem rather rapid but not knowing your exact system it may be ok.  I would think you have an expansion tank after the boiler and then circulators off that tank.  So many questions, how old is the system and were the controls ever readjusted. OK I'll stop now.

       Paul B      

[Kiwi_Roy]

Yes as you say there is a circulation tank. Build year 2011 ,somewhere close to that.
The 13.5 Volts I see is on the main valve, it comes on at the same time the pilot gets 25 then when the pilot lights it jumps to 25.
A 3/4" valve.
The control wiring looks a little hairy, for some reason the High Limit thermostat on one side was bypassed in favour of one installed at the discharge nozzle, the original high limit thermostat which had a capillary run to the discharge nozzle has been replaced by another right at the nozzle, the wiring of that is definitely not to code, what looks like telephone wire is used to reach around the back-side, its only 24 Volts of course but even so.
I doubt its ever been set-up properly, nothing in the log book since 2013
I pulled the gas trap off, nothing inside there, it definitely would have caught any liquids.
In light of the rapid cycling i'm wondering if the valve is just not worn out, I did pull the pilot flame out the other day, while it's buzzing the flame
Currently it comes on buzzes for about 10 seconds then lights, I will keep my fingers crossed.

The other day I took the pilot out, while the valve is buzzing any flame is really wispy obviously getting some gas (chattering I guess) but not enough for a decent flame, good spark. When it finally clicks open a good flame.
Thanks again for your help

I looked at the high limit thermostat its set at ~140 with 5  degrees F differential, I tried increasing that to 10 but it doesn't seem to change much, perhaps runs a little less frequent. I'm guessing it has tons of over capacity.

[RinkRat II]

      Something comes to mind on the cycling,  there should be a limit switch on the tank that actually tells the controller to light the boiler if all the other switches are calling for heat. This may be maladjusted and causing only the boiler switches to control the valve in too rapid a succession.

    Paul B

[Kiwi_Roy]

That makes sense to me, I will look in the morning. Yes it has a thermostat in the tank, around the back unreachable.
I was wondering if the circ. pump was doing its job properly i.e. getting the heat out of the boiler quick enough. It's definitely spinning and the flow switch is closed.

[LaMojo]

Does it have an optical sensor for flame detection?

[Kiwi_Roy]

Does it have an optical sensor for flame detection?

No, the spark wire is in the flame and when not sparking it rectifies a tiny current, I haven't seen an optical unit in eons.

[LaMojo]

No, the spark wire is in the flame and when not sparking it rectifies a tiny current, I haven't seen an optical unit in eons.

The confusion is indicating the unit you have is a boiler but it doesn’t generate steam so therefore it is classified as a water heater. Big difference in the two. (i should have looked up the unit in question before responding) And no, it wouldn’t have an optical flame sensor. Those are reserved for large boilers used in central energy plants to supply multiple sections or buildings on campus.

[Kiwi_Roy]

Yes, AO Smith call it a boiler operates about 150 F but as you say its nothing like the boilers I worked on at hospital / power station.
I also worked at a fairly large university for a while, the heating was done by central High Temperature hot water, no steam there either but way above boiling point and supposed to be more efficient. they certainly looked like boilers but I guess if it's not boiling eh!